Removal of NOx from exhaust streams is a critical technology for the automotive and electric power industry in meeting ever more stringent air emissions standards. In view of future emissions standards automotive companies have focused on the design of lean burn engine designs where the combustion fuel to air ratio is substantially less than the stoichiometric ratio used in present day engines. Under lean burn conditions, the existing catalytic converter technology does not work. Thus, new catalytic converters must be developed for lean burn engine technology to be implemented. Lean burn deNOx catalysts are highly sought after and are the focus of considerable research worldwide. A rather narrow window of satisfactory operating temperatures has characterized most catalysts for lean burn applications. Specifically, they only effectively convert NOx over small temperature ranges that do not always match the temperatures at which the NOx is emitted. Some of the better catalyst materials have included metal-substituted zeolite catalysts such as Cu-ZSM-5 and related catalysts consisting of various zeolites with metal ions substituted into the zeolite structure. These materials offer activity at higher temperatures than conventional platinum-based deNOx catalysts, but the best operating temperature ranges are generally too high (above about 400° C.) and too narrow (only about 100° C. in effective range width).
In addition, internal combustion engines emit a large amount of unburned hydrocarbons during cold engine start-up. In fact, a rather large fraction of the total emitted hydrocarbons released during the first minutes of engine operation are due to the uncombusted hydrocarbons in the rich fuel mixture. Such release of hydrocarbons after engine start-up poses a special problem, as at that point the temperatures of the exhaust gas and the catalytic converter are generally not high enough for conversion of the gaseous pollutants by conventional catalysts. The catalysts in present catalytic converter systems are generally ineffective at ambient temperatures and must reach high temperatures, often in the range of 300° C. to 400° C. before they become effective.
For example, U.S. Pat. No. 5,171,553 describes the catalytic decomposition of N2O from gaseous mixtures, but conversion or decomposition rates of 50 percent of the N2O are shown as requiring temperatures of greater than about 275° C. as shown in their FIG. 3 for rhodium-exchanged zeolites and of greater than about 350° C. as shown in their FIGS. 1 and 2 for copper- and cobalt-exchanged zeolites.
U.S. Pat. No. 5,776,423 describes the catalytic decomposition of NOx from gaseous mixtures, but shows conversion or decomposition rates of less than 50 percent of the NOx at temperatures of less than about 375° C. even before aging of the catalyst as shown in their FIG. 1.
Numerous other patents such as U.S. Pat. No. 5,935,529, U.S. Pat. No. 5,869,013, U.S. Pat. No. 5,834,395, U.S. Pat. No. 5,695,728, U.S. Pat. No. 5,449,504, U.S. Pat. No. 5,443,803, U.S. Pat. No. 5,427,753, U.S. Pat. No. 5,358,916, U.S. Pat. No. 5,260,043, and U.S. Pat. No. 5,171,553, either describe the problems of treating exhaust gases at temperatures below 300° C. to 400° C. or fail to show conversion rates of 50 percent or more at temperatures below about 300° C. to 400° C.
Catalysts have now been found which overcome these obstacles and provide for effective NOx conversion at lower temperatures and a wider temperature range.
It is an object of this invention to provide a process for NOx conversion in an exhaust stream under temperature conditions of from about 200° C. to about 600° C.
Another object of this invention is a process for NOx conversion of at least about 60 percent in an exhaust stream under temperature conditions of from about 200° C. to about 600° C., especially from about 200° C. to about 400° C.
Yet another object of the present invention is to provide for NOx conversion in an exhaust stream with a broad effective operating range, i.e., a high conversion rate such as at least about 60 percent for a temperature range of greater than 200° C., preferably greater than 300° C.
Still another object of the present invention is to provide compositions of matter useful for the catalytic reduction of nitrogen oxides in an exhaust stream under temperature conditions of from about 200° C. to about 600° C., especially from about 200° C. to about 400° C.